Multifrequency antenna array



March 8, 1955 G. N. cARMlcHAEL 2 ,703,840

MULTIFREQUENCY ANTENNA ARRAY Filed Feb. 9, 1951 1 o O .96 I Zy J X50 X44 i I l INVENTOR. 62/5/70; M Camr/mae/ HTTORNEK United States Patent MULTIFREQUENCY ANTENNA ARRAY Gershom N. armichael, Griggsville, Ill.

Application February 9, 1951, Serial No. 210,108

Claims. (Ci. 250-3353) This invention relates to antenna structure of the kind having both active and parasitic elements, the primary object being to provide optimum gain on any of a number of radio frequencies through advantageous use of all of the remaining elements in the array whenever any one element is active on its particular frequency.

The use of parasitic elements in antenna arrays as directors and reflectors to provide optimum gain and minimum interference in an active element on a particular frequency, is well known. Such parasitic elements, however, serve no other purpose so far as becoming active on other frequencies. active element, in conventional structures, is provided with its own set of parasitic elements and even when the latter are rendered common to a number of active elements, an expensive, cumbersome and inefficient antenna system must be provided.

It is the most important object of this invention, therefore, to provide a number of active elements in a single array, so interconnected electrically as to render each alternately parasitic to the other, depending on which is active, not only from the standpoint of providing additive voltage directly, but from the standpoint of serving in a reflection and/or directive capacity.

Another important object of this invention is the provision of antenna structure wherein the elements, when operating parasitically, provide voltage gain for an active element by direct connection therewith and with the feed line through proper phasing relationships.

- A further important object of this invention is to provide an antenna array having a number of elements each of a length corresponding to a respective frequency, critically spaced and interconnected with proper proportions and electrical distances with a common feed line, to effect the necessary phasing for accomplishing the above mentioned results relative to gain and output voltages.

' It is an object of this invention to provide an antenna array which can be used for reception or transmission on either'of two or more predetermined frequencies and which requires no manual adjustment at the antenna toh accomplish a change from one frequency to the ot er.

Many other minor objects, including details of construction will be made clear or become apparent as the following specification progresses, reference being had to the accompanying drawings, wherein:

Figure 1 is a top plan view of a multi-frequency'array made according to my present invention.

Fig. 2 is a side elevational view thereof.

Fig. 3 is a graphic representation of the voltage patterns for two selected frequencies.

Previously, it has been necessary to provide a separate antenna array for each frequency employed. Such a requirement has made operation on more than one frequency prohibitive to many users because of the cost and difficulty of installation of separate antennas. It is natural to consider the possibility of one conventional array having sulficiently broad frequency response to cover two adjacent frequencies, but the experiments in tuning the elements to obtain this result have not been successful. Since the functioning of the parasitic elements is dependent on dimensions and spacing of such elements to provide the proper phasing, it is not possible to have characteristic parasitic behavior over a range of frequencies which is any considerable percentage of the fundamental frequency.

Accordingly, each 1 1 "ice Nearly all of the properties possessed by an antenna as a radiator or transmitter also apply when it is used as a receiving antenna. Current and voltage distribution, impedance and resistance, and directional characteristics are the same in a receiving antenna as they would be if it were used as a transmitting antenna. This reciprocal behavior makes possible the design of a receiving antenna of optimum performance based on the same considerations going into the design of a transmitting antenna. Accordingly, as will hereinafter become apparent, in describing the antenna structure hereof, it is to be understood that the array may be used within the broad concepts of the invention with equal advantages either for transmitting or receiving radio frequency signals. Additionally, while the novel features oi the array have been developed primarily because of the dire need thereof in the held of television, it is not limited to such use and may have tremendous importance to the radio iield, as will become clear to those skilled in the art.

le'or purposes of description and illustration, a four element array is shown. However, it is to be understood that this invention is not to be limited as to the number of elements employed, since anyone skilled in the art is capable of adding elements to amplity the signal received or transmitted.

ill the IOllOWillg description of such an antenna, when used I01 reception, an active element shall be understood to be an element which is connected to the feedllne so that the voltage induced in it is delivered directly to the teed-line. A parasitic element shall be understood to be an element which re-radlates its induced voltage in such a way that voltage phases will produce a desired result in the active element, such as addition, in the case or a director, and cancellation or re ection, in the case of a reflector. The functioning or a parasitic element as a renector or as a director is determined by its physical d'lmenslons and spacing from the active element.

A parasitic array, in general, consists of an active element, together with one or more parasitic elements, designed to deliver a voltage by means or a teed-line to some certain point. The parasitic elements are designed to provide gain 10); signals from one direction and rejCCtlOll of signals from some other direction, these elements being designed ordinarily to provide gain in one direction and re ection from the opposite direction. ln general, in such a parasitic array, the forward gain and oacltward re ection can be maintained only over a very narrow band or frequencies. An array made in accordance with the principles hereof is, however, operative on two or more or such narrow band of frequencies.

Basically, the principle involved calls for a single element in an array to ILUJCHOD in a dual way, both as an active element on one frequency and as a parasitic element on a dlnerent frequency. ln the simplest case, such an array would consist of two elements, one of which acts as an active element on a frequency, 71, while the other acts as a parasitic element on that frequency. Uh some other frequency, is, the first element would act as a parasitic element, While the second would be the active element for the frequency, h. This is possible since the functioning as a parasitic element necessitates a length dilferent from that of an active element. in this case, each of the elements is also an active element and it is necessary to connect each to the feedline. This means that the two elements have a direct connection to each other, and this connection must be made in such a way that the voltages, both from the direct connection and from the re-radiated signal, will have the proper phase relation.

The antenna array chosen forillustration in Figs. 1 and 2 of the drawing, is broadly designated by the numeral 19 and includes an elongated supporting bar 12 that is horizontally disposed when the array 10 is' used in one common manner. The supporting bar 12 is secured intermediate its ends to a vertical mast or standard 14.

The array 10 illustrated is provided with four elements 16, 18, 20 and 22. The elements 18 and 20 being known in the trade as folded dipoles. It is noted that the dipoles 18 and 20 are of ditfering lengths, that the element 16 is longer than the dipole 18 and that the element 22 is shorter than dipole 20. It is well known that such lengths are critical, and, in the instance shown, the length of dipole 18 has been chosen to receive or transmit radio signals having a frequency of 6672 megacycles, while the length of dipole 20 has been chosen to receive or transmit on 76-82 megacycles. Likewise, the lengths of elements 16 and 22 should be chosen to render the same operative as a reflector and as a director respectively for the frequency ranges of the two primary elements 18 and 20. Such precise physical lengths vary directly with the frequencies employed and are well known to those skilled in this field.

Thus, in the illustrated antenna 10, dipole 18 is 80 inches long, dipole 20 is 69 inches long, reflector 16 has a length of 85 inches, and director 22 is preferably 66 inches long.

Each dipole 18-20 includes a pair of spaced-apart elongated, preferably tubular members 24 and 26 respectively, of metallic or other conducting material, together with a center member of the same length in spaced parallelism with the outermost members 24 and 26, as the case may be. In a folded dipole such center member consists of a left segment 28 and a right segment 30 for element 18, as well as a left segment 32 and a right segment 34 for the element 20. of each dipole 1820 are interconnected electrically at the outermost ends in any suitable manner such as by metallic plates 36. Proper operation demands, however, that the left and right segments be electrically separated at their proximal ends and thus there is provided in the present construction, tubular insulators 38 telescopically receiving the segments and serving as a means of joinder thereof to the bar 12.

Following the principles of this invention. the center segments of the dipoles 18 and 20. must be joined with each other electrically and with a feed-line (not shown) whether the latter serves to supply voltages to a receiver or to receive voltages from a transmitter. To this end, a terminal bar 40 of insulating material is secured to bar 12 between the elements 18 and 20 for mounting a pair of spaced terminal posts 42 and 44, one conductor of the feed-line being joined to each post 42-44 respectively.

Each segment 283032 34 is provided with a conductible clamp 46 adjacent the corresponding tube 38 serving as a means for joining such segments with the posts 42 and 44 and thus with the feed-line. ductor 48 joins segment 30 with post 42; a conductor 50 connects segment 28 and post 44; a conducting line 52 is attached to segment 34 and to post 44; and a fourth conductor 54 joins the segment 32 with the post 42. It is thus seen that, in the illustrated array 10, conductors 52 and 54 are transposed between element 20 and the feed-line connected to posts 42 and 44.

As above indicated, the purposes of such arrangement include rendering the elements 13 and 20 alternately active on their respective frequencies within a single bay. However, by following certain important considerations. the other element is not completely inactivated, but serves to provide an appreciable voltage gain for the active element, not only through parasitic functioning. but by direct inducement to the feecl-line or, in the case of use with a transmitter, to the atmosphere. It is thus clear that in order to render the elements 18 and 20 mutually cooperative in this respect, a proper phasing relationship must be established therebetween.

With the lengthsof dipoles 18 and 20 chosen for the above mentioned frequencies, it has been found preferable to space the same at a distance equal to one-tenth of the average of the wave lengths of dipoles 18 and 20. Accordingly, the distance between the center segments of dipoles 18 and 20 is approximately 22 inches.

The spacing and lengths of the elements 16 and 22 which are purely parasitic are designed to provide the best compromise between three primary objectives, i. e., high forward gain, broad frequency response, and high front-to-back ratio. To this end, the distance between reflector 16 and the center element of dipole 18 should be equal to approximately one-tenth of the wave length of the latter or substantially 25 inches. The same proportion has been found preferable in establishing the A con- The three members distance between the center segment of dipole 20 and director 22 or approximately 20 inches.

All of the above dimensions may be varied within virtually infinite ranges but with the distance between the dipoles chosen, proper phasing can be establishing only by effecting a proper ratio of electrical lengths between the dipoles through conductors 48, 50, 52 and 54. In the present instance, the electrical distance from the outermost end of segment 34 (adjacent its plate 36) to its clip 46 and thence through conductor 52 to post 44 is equal to the electrical distance from the outermost end of segment 30 to post 42 through conductor 48. Likewise, the electrical distance from the outer end of segment 32 through conductor 54, to post 42, is equal to the electrical distance from the outermost end of segment 28 to post 44 via conductor 50. Such 1 to 1 ratio varies directly with the chosen distance between the dipoles and even with the precise location of the terminal posts 42 and 44 relative to the dipoles. In the present antenna, such posts are co-planar with the dipoles and spaced approximately 7% inches from the segments 28-30.

It is Well appreciated in this field that no precise formula can be set forth for establishing the proper phasing relationship produced by the dimensions and ratios above set forth. Thus, changing of the distance between the dipoles may require one or more additional alterations such as varying the electrical distance ratio above set forth, or re-positioning the terminal posts 42-44 toward or away from the dipole 18 or in another plane.

Such factors as the diameters of the members forming a part of the dipoles, the widths thereof, the electrical resistance of the interconnecting conductors, and so forth, may also affect the desired phasing characteristics. To this end, it is also recognized that in some constructions, the transposition between conductors 52 and 54 must be eliminated to effect the results of the present invention.

Extensive tests have proved that the operation of antenna array 10 is substantially as follows:

Assuming first that dipole 18 is rendered active on its particular frequency, even at maximum efficiency, it will deliver little more than fifty percent of the induced voltage to the feed-line through posts 42 and 44. Since an active element has some of the necessary characteristics of a parasitic element, the remaining voltage is, in a large part, re-radiated. Such re-radiated voltages are directed to a considerable extent to the inactive dipole 2t), and induced thereby through segments 3234 and conductors 5254 to the feed-line to provide gain in the output voltage of dipole 18. In addition, added voltages on the frequency of dipole 18 are received directly by the dipole 20 and fed to the feedline to provide additive effect.

Such operation on the part of inactive dipole 20 is made possible solely because of the fact that proper phasing is provided in theconnection of the dipoles with each other and with the feed-line while maintaining the critical values above described. In absence of a proper choosing of the distance between the dipoles, the parasitic effect would be seriously affected. And, without proper phasing, the voltages received by dipole 20 either directly or by re-radiation from dipole 18 would not produce the desired gain in the feed-line.

It is seen therefore, that when dipole 18 is active, its operation is enhanced not only by element 16 operating as a reflector and element 22 as a director, but by the dipole 20 also operating as a director but inducing its received voltages directly to the feed-line.

Conversely, when the dipole 20 is active on its frequency, the dipole 18 operates parasitically as a reflector for cancelling undesired signals from other directions. However, in such instance, the dipole 18 receives voltages that are re-radiated by dipole 20 and also receives directly voltages corresponding to the frequency of dipole 20, both of which are impressed upon the feed-line to provide a very significant and extremely important additive effect.

It can now be understood why the precise physical characteristics of antenna 10, as illustrated in the drawmg have no importance whatever to the principles involved herein. The new departure from conventional parasitic antennas contemplates two or more active elements, whether or not the same are formed as dipoles and whether or not the additive elements 16 or 22 are utilized. In its simplest form, a pair of active elements such as segments 30 and 34 mounted on a suitable support 12, may be coupled together and to a feed-line irrespective of the position of the point of connection with the feed-line, in which case such active elements would most likely be arranged vertically.

Another possibility, still within the principles hereof, would contemplate the elimination entirely of members 16, 22, 24 and 26 from the array illustrated in Figs. 1 and 2.

Still further, a virtually unlimited number of active elements such as dipoles 18 and 20, could be provided, each operating in an additive way when any one element is active.

Finally, the number of reflectors and directors may be varied as desired.

Antenna 10, therefore, is characterized by its high gain, sharp lobe pattern, high front-to-back ratio and low vertical wave angle response. Figure 3 of the drawing shows graphically voltage lobes 56 and 58 for the frequencies of dipoles 18 and 20 respectively, it bemg noted that the frontto-back ratio is high and remains above 20' decibels from the carrier wave for the frequency of dipole 18 through the carrier frequency of dipole 20.

Through use of the antenna structure hereof, the

problem of attempting to produce a single bay having sutficiently broad frequency response to cover two or more adjacent frequencies is overcome. Irrespective of the fact that the functioning of parasitic elements is dependent upon dimensions and spacing to provide proper phasing, rendering cost and installation difficulties prohibitive, particularly in fringe areas, in order to cover a wide range of frequencies, following the principles of this invention affords excellent parasitic behavior in a single bay.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. A multielement, multifrequency, unidirectional, broadside antenna array adapted for high gain operation selectively on any one of a number of separated, distinct frequency channels, throughout the respective band widths thereof, with each channel centered about a single predetermined frequency, by minimizing losses of reradiated energies, and notwithstanding any inherent impedance mismatching resulting from different self-impedances of the elements, said single predetermined frequencies being different and separated, said antenna array comprising a plurality of antenna elements, each of said elements being self-resonant to a dilferent one of said single predetermined frequencies and the elements progressively decreasing in electrical length as one end of the array is approached, whereby the frequencies to which the same are tuned are progressively higher as said one end of the array is approached, each element having conductor means coupled thereto; structure mounting said elements against relative movement and in predetermined spaced relationship, whereby to electromagnetically couple each element with the remaining elements and thereby render each a parasitic element at the resonant frequencies of the remaining elements in order to utilize a substantial portion of said reradiated energies; and transmission line terminal means coupled with said conductor means to render each of said elements a driven element on its respective resonant frequency.

2. A multielement, multifrequency, unidirectional, broadside antenna array adapted for high gain operation selectively on any one of a number of separated, distinct frequency channels, throughout the respective band widths thereof, with each channel centered about a single pre determined frequency, by minimizing losses of reradiated energies, and notwithstanding any inherent impedance mismatching resulting from different selfdmpedances of the elements, said single predetermined frequencies being different and separated, said antenna array comprising a plurality of antenna elements, each of said elements being self-resonant to a different one of said single predetermined frequencies and the elements progressively decreasing in electrical length as one end of the array is approached, whereby the frequencies to which the same are tuned are progressively higher as said one end of the array is approached; structure mounting said elements against relative movement and in predetermined spaced relationship, whereby to electro-magneticallycouple each element with the remaining elements and thereby render each a parasitic element at the resonant frequencies of the remaining elements in order to utilize a substantial portion of said reradiated energies; transmission line terminal means; and means for rendering each of said elements a driven element on its respective resonant frequency and comprising conductor means for each element respectively, coupling the elements with the transmission line terminal means and provided with predetermined electrical lengths for delivering voltages carried thereby in phase.

3. A muitielement, multifrequency, unidirectional, broadside antenna array adapted for high gain operation selectively on any one of a number of separated, distinct frequency channels, throughout the respective band widths thereof, with each channel centered about a single predetermined frequency, by minimizing losses of reradiated energies, and notwithstanding any inherent impedance mismatching resulting from different self-impedances of the elements, said single predetermined frequencies being dilferent and separated, said antenna array comprising a plurality of elongated antenna elements having parallel, longitudinal axes and median, transverse, aligned axes, said axes all being in a common horizontal plane, each of said elements being self-resonant to a different one of said single predetermined frequencies and the elements progressively decreasing in length as one end of the array is approached, whereby the frequencies to which the same are tuned are progressively higher as said one end of the array is approached, each element having a pair of colinear, quarter-wave segments, each segment having a conductor coupled thereto and disposed to render each element a center-fed, half-wave dipole; structure mounting said elements against relative movement and in predetermined spaced relationship, whereby to electro-magnetically couple each element with the remaining elements and thereby render each a parasitic element at the resonant frequencies of the remaining elements in order to utilize a substantial portion of said reradiated energies; and a pair of spaced, transmission line terminals spaced from said elements and connected directly with said conductors to render each of said elements a driven element on its respective resonant frequency.

4. A dual element, dual frequency, unidirectional, broadside antenna array adapted for high gain operation alternately on either of a pair of separated, distinct frequency channels, throughout the respective band widths thereof, with each channel centered about a single, predetermined frequency, by minimizing losses of reradiated energies, and notwithstanding any inherent impedance mismatching resulting from different self-impedances of the elements, said single predetermined frequencies being different and separated, said antenna array comprising a pair f elongated antenna elements having parallel, longitudinal axes and median, transverse, aligned axes, said axes all being in a common plane, each of said elements being self-resonant to a different one of said single predetermined frequencies, one element being longer than the other, whereby the frequency to which it is tuned is lower than the frequency to which said other element is tuned, each element having a pair of colinear, quarter-wave segments; structure mounting said elements against relative movement with the shorter element ahead of the longer element and in predetermined spaced relationship, whereby to electro-magnetically couple each element with the other and thereby render the shorter element a parasitic director for the longer element at the resonant frequency of the latter and render the longer element a parasitic reflector for the shorter element at the resonant frequency of the latter in order to utilize a substantial portion of said reradiated energies; a pair of spaced, transmission line terminal means spaced from said elements; and means for rendering each of said elements a driven, center-fed, half-wave dipole on its respective resonant frequency and comprising conductor means for each element respectively, coupling the elements with the transmission line terminal means and provided with predetermined electrical lengths for delivering voltages carried thereby in phase.

5. A dual element, dual frequency, unidirectional, broadside antenna array adapted for high gain operation alternately on either of a pair of separated, distinct frequency channels, throughout the respective band widths thereof, with each channel centered about a single, predetermined frequency, by minimizing losses of reradiated energies, and notwithstanding any inherent impedance mismatching resulting from different self-impedances of the elements, said single predetermined frequencies being different and separated, said antenna array comprising a pair of elongated antenna elements having parallel, longitudinal axes and median, transverse, aligned axes, said axes allbeing in a common horizontal plane, each of said elements being self-resonant to a difierent one of said-single predetermined frequencies, one element being longer than the other, whereby the frequency to which it is tuned is lower than the frequency to which said other element is tuned, each element having a pair of colinear, quarter-wave segments, each segment having a conductor coupled thereto and disposed to render each element a center-fed, half-wave dipole; structure mounting said elements against relative movement with the shorter element ahead of the longer element and in predetermined spaced relationship, whereby to electro-magnetically couple each element with the other and thereby render the shorter element a parasitic director for the longer element at the resonant frequency of the latter and render the longer element a parasitic reflector for the shorter element at the resonant frequency of the latter'in order to utilize a substantial portion of said M radiated energies; and a pair of spaced, transmissionline terminals spaced from said elements and connected directly with said conductors to render each of said ele ments a driven element on its respective resonant frequency, the conductors of the shorter element being longer than the conductors of the longer element, the electrical length of each segment of the longer element plus the electrical length of its conductor being substantially the same as the electrical length of each segment of the shorter element plus the electrical length of the con doctor of the latter.

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